1. Field of the Invention
The present invention relates to optical networks and more specifically to techniques for protecting optical physical links using redundant protection channels.
2. Description of the Related Art
Optical transmission systems, such as those using Dense Wavelength Division Multiplexing (DWDM), provide extremely wide bandwidth for communications. Each DWDM transmission system carries a plurality of optical channels (wavelengths) on each optical fiber and through each optical repeater. However, there is a trade off between the lower cost of transport provided by wider bandwidth communications channels and their vulnerability to a large-scale disruption of communications services because of a transmission equipment and/or medium failure. It is, therefore, important that DWDM optical transmission systems have the capability to quickly recover from such transmission failures.
Protection of optical networks in the event of failures (e.g., fiber cuts, transmitter failure, and amplifier instabilities) typically involves redirecting the service traffic from a channel on the optical fiber within which it was originally carried (i.e., the service channel, denoted by S) that has been affected by the failure to another unaffected source of bandwidth (i.e., the protection channel, denoted by P) whereby the service traffic may ultimately reach its intended destination.
Typically, optical switches located within a node are used to accomplish this redirection. For example, it is typical to direct optical signals transmitted from edge equipment along one direction on the network (e.g., East) to another (e.g., West). In a ring, mesh, hypercube, or other redundantly connected optical network topology, performance monitoring that analyzes and monitors the traffic on S and P at the various destination and intermediate nodes can be used by a microcontroller to autonomously switch over to a protection channel or path P by sensing a failure on the primary service path S. Note that the protection channel P can be the same or different optical wavelength (i.e., wavelength diversity), but it is typically on a different fiber, and that fiber is typically carried in a different bundle along a unique path from the first (i.e., path diversity).
There are a number of different optical protection schemes in use today that build upon this basic principle. These include 1+1 protection, span protection, 1:1 protection, and shared protection. These schemes are described in detail in Al-Salameh, D. Y., Korotky, S. K., Levy, D. S., et al., Optical Fiber Telecommunication—Volume IVA, Elsevier Science, USA, Ch. 7, pp. 318–327, incorporated herein by reference. Additional shared optical protection schemes denoted 1:N are discussed in detail in U.S. patent application Ser. No. 09/675,733 filed on Sep. 30, 2000 as attorney docket no. Al-Salameh, D. Y., 10-1-2-5-35, also incorporated herein by reference.
It is a generally accepted practice to provide a continuous or “keep-alive” signal to the protection channel P to allow the system to determine that P is alive and alarm free (i.e., kept alive) prior to a given failure event. Keep-alive signals can be provided in numerous ways; however, it is typical to use a fairly accurate copy of the service signal as the keep-alive source, and it is typical of all of the schemes referenced above to derive this copy via an optical splitting function of some nature.
There are two basic schemes in use today for modulating a light signal with data. The first scheme, termed “direct modulation” involves the application of the data or modulation signal directly to the laser source, essentially switching the laser on and off corresponding to a modulating data stream of logical “1”s and “0”s. This scheme suffers from instability in the transmission wavelength of the laser referred to as “chirp” and related transient effects that result from the direct amplitude modulation of the laser. The second and generally preferred scheme for optical modulation is termed “external modulation.” In this preferred scheme, the laser is driven at a constant power level and the resulting continuous wave (CW) output of the laser is fed to an “external modulator” such as a Mach-Zehnder (MZ) device.
Thus a typical optical transmitter configuration is a CW laser followed by an MZ external modulator and, in protected optical networks, it is typical to follow this configuration with an optical splitter to generate the signals that will supply light to the service S and protection P channels.
Use of an optical splitter to generate the keep-alive signal has the inherent disadvantage of introducing a splitter loss (e.g., ˜3.5 dB) into the signal path that may result in higher system costs to overcome (e.g., additional optical amplifiers in the path, higher-cost transmitter lasers, or more-expensive low-loss components in the transmitter or optical pathways to save power budget). As an alternative to an optical splitter, a network's transmission equipment (e.g., an optical translation unit (OTU)) can be designed to have an extra transmitter that serves the keep-alive function. However, such a device is expensive due to the cost of the high-speed optoelectronics needed in the extra transmitter. Optionally, a single-channel OTU in the line system can be designed (i.e., programmed) to transmit a keep-alive signal when it is not being fed by an input signal. This approach is still burdened with the cost of the additional OTU hardware and requires intelligence in the OTU and complex control algorithms to distinguish between transients on the line system and actual failures.